Well I think it'd be cool to have:
MDK 3
This probably won't happen because BioWare is too busy with another certain franchise to be bothered with awesome things like 6-armed cigar smoking robot dogs and radioactive toast.
Perfect Dark 2
No shitty prequel but a proper sequel to the awesome N64 game. Get on that, Rare.
Marathon 4
I don't think Bungie is going to decide "Hey, let's do a sequel to another space marine saga that we made back in the '90s. Still, with today's graphics and voice acting the story would make your head explode. Hell, even back then it almost did.
Not a sequel, but I'd still really like to have StarCraft: Ghost. If Blizzard decided to try another genre like a TPS platformer, I think it'd be awesome. However, they still have two StarCraft 2 installments to put out, and have to keep the MMO zombies suckling at their teat, so I don't see it happening anytime soon.
A spiritual successor to Quake by id Software.
The original Quake wasn't much like its successors. The gameplay was similar but the settings were completely different. Quake 2-4 had a space marine sci-fi setting, whereas the first took place primarily in a weird Lovecraftian nightmare realm. Not hellish like Doom, just... bizarre. The game couldn't really be part of the Quake series but I'd love to see them revisit that aesthetic.
Well, I'm getting more Duke Nukem, the Alice sequel and another BioShock is on the way so other than that I'm a pretty satisfied on the gaming side of my life right now.
EDIT: Oh, oh, I got one more; another Syndicate game. Anyone remember those? Anyone? [small]*sigh*[/small]